Linear motor

ABSTRACT

When a pitch between centers of two adjacent magnetic pole portions disposed on an armature is defined as τs, the length W of permanent magnets disposed in the middle is equal to the pitch τs (W=τs). The length of each of the two end permanent magnets respectively disposed at either end of a permanent magnet magnetic pole row is denoted by W 1 . A ratio w1/W of the length W 1  to the length W is defined as 0.43&lt;W1/W&lt;0.49. With this definition, cogging torque of a linear motor may sufficiently be reduced.

FIELD OF THE INVENTION

The present invention relates to a linear motor. BACKGROUND OF THEINVENTION

Japanese Patent Publication No. 11-308850 discloses a linear motor whichincludes a stator and a movable element so that the movable element mayreciprocate with respect to the stator, and further includes an armaturedisposed on one of the stator and the movable element and a permanentmagnet magnetic pole row disposed on the other of the stator and themovable element. The armature of the linear motor has an iron corehaving three or more magnetic pole portions arranged along a movingdirection of the movable element, and armature windings of three phasesrespectively wound around the three or more magnetic pole portions ofthe iron core. The permanent magnet magnetic pole row has a frame of anonmagnetic material and a plurality of permanent magnets disposed inthrough-holes formed in the frame. The plurality of permanent magnetsare disposed at a given interval in a row along the moving direction ofthe movable element so that north poles and south poles may alternatelybe arranged. The permanent magnet magnetic pole row faces the magneticpole portions with a given gap therebetween. To reduce the coggingtorque, in this linear motor, a length W1, as measured in the movingdirection, of each of two permanent magnets which are respectivelydisposed at either end of the permanent magnet magnetic pole row isdefined to be less than a length W, as measured in the moving direction,of the remaining permanent magnets, and that a ratio W1/W of the lengthW1 to the length W is defined as 0.5<W1/W<0.6. In addition, when a pitchbetween centers of two adjacent magnetic pole portions of the pluralityof magnetic pole portions constituting the armature is defined as τs,and a pitch between the center of one of the two permanent magnetsrespectively disposed at either end of the permanent magnet magneticpole row and the center of the permanent magnet adjacent thereto isdefined as λ′, a ratio λ′/τs of the pitch λ′ to the pitch τs is definedas 0.9<λ′/τs<1.0.

However, it is not possible to sufficiently reduce the cogging torque inconventional linear motors.

SUMMARY OF THE INVENTION

It is therefore an object of the present invention to provide a linearmotor in which the cogging torque can sufficiently be reduced.

A linear motor, of which improvement is aimed by the present invention,includes a stator and a movable element. An armature is disposed on oneof the stator and the movable element. The armature includes an ironcore having three or more magnetic pole portions arranged along a movingdirection of the movable element and armature windings of three phaseswound around the iron core to excite the three or more magnetic poleportions. A permanent magnet magnetic pole row is disposed on the otherof the stator and the movable element. The permanent magnet magneticpole row includes a base made of a magnetic material and a plurality ofpermanent magnets disposed on the base at a given interval in a rowalong the moving direction so that north poles and south poles mayalternately be arranged. The plurality of permanent magnets are arrangedto face the three or more magnetic pole portions with a given gaptherebetween. A length W1, as measured in the moving direction, of eachof two permanent magnets which are respectively disposed at either endof the permanent magnet magnetic pole row is less than a length W, asmeasured in the moving direction, of the remaining permanent magnets.The number of slots per pole per phase is ⅜, three-eighths. Here, thenumber of slots per pole per phase is defined as a value obtained bydividing the number of slots of the armature by the number of poles ofthe permanent magnet magnetic poles, and further dividing the quotientby the number of phases of the armature windings.

In one aspect of the present invention, when a pitch between centers oftwo adjacent magnetic pole portions of the plurality of magnetic poleportions constituting the armature is defined as τs, the length W of theremaining plurality of permanent magnets is defined as W=τs, and a ratioW1/W of the length W1 to the length W is defined as 0.43<W1/W<0.49.After keenly studying, the inventors of the present invention have foundthat the cogging torque can be reduced to a practically acceptable levelby defining the ratio W1/W as 0.43<W1/W<0.49. It may be considered thatthe cogging torque may effectively be negated due to the arrangement ofpermanent magnets as described above, namely, when the ratio W1/W isdefined within the above-mentioned range. When the ratio W1/W is 0.43 orless, or the ratio W1/W is 0.49 or more, the cogging torque increases toan unacceptable level or beyond an allowable level. The presentinvention is based on the discovery that the cogging torque takes theminimum value when the ratio W1/W falls within the above-mentionedrange.

In another aspect of the present invention, a linear motor may include aplurality of permanent magnet magnetic pole rows. When a plurality ofthe permanent magnets disposed at one side ends of the plurality ofpermanent magnet magnetic pole rows are different in length from oneanother as measured in the moving direction and an average length of thepermanent magnets disposed at the one side ends is defined as W10, andwhen a plurality of the permanent magnets disposed at the other sideends of the plurality of permanent magnet magnetic pole rows aredifferent in length from one another as measured in the moving directionand an average length of the permanent magnets disposed at the otherside ends thereof is defined as W20, the permanent magnet magnetic polerows may be configured so that a ratio W10/W of the average length W10to the length W is 0.43<W10/W<0.49 and a ratio W20/W of the averagelength W20 to the length W is 0.43<W20/W<0.49. Within the above-definedratios, the present invention is then applicable.

Preferably, a spacer, which is made of a nonmagnetic material, isdisposed between two adjacent permanent magnets of the plurality ofpermanent magnets to form a space therebetween. In this manner, thepresence of the spacer can prevent electrical short between two adjacentpermanent magnets.

The spacer may be made by cutting a conventional wire material at lowmanufacturing cost.

In a further aspect of the present invention, the permanent magnetmagnetic pole row may be disposed on the movable element and thearmature may be disposed on the stator. In this arrangement, the basemay be formed in a cylindrical or columnar shape and the plurality ofpermanent magnets may be fixed onto an inner peripheral surface of thebase. The stator may partially be arranged inside the movable element.The armature may be fixed onto a base station via a stator fixture atboth ends thereof in the moving direction of the movable element. Aguide block may be fixed to the movable element and the guide block mayslidably be supported by a guide rail disposed on the base station.Thus, the movable element is capable of moving in the moving directionwith respect to the stator as the guide block slides on the guide rail.With this arrangement, the movable element can be moved with respect tothe stator with a simple configuration.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a cross-sectional view of a linear motor according to oneembodiment of the present invention.

FIG. 2 is a partially enlarged view of FIG. 1.

FIG. 3 is a graph showing the relationship between cogging torque and aratio W1/W of a linear motor used in the test.

FIG. 4 is a graph showing the relationship between cogging torque anddisplacement for a movable element of the present invention as well asfor a linear motor of a comparative example, wherein the ratios W1/W aredifferent in these linear motors.

FIG. 5 is an exploded plan view showing half (four) of eight permanentmagnet magnetic pole rows according to another embodiment of the presentinvention.

FIG. 6 is a partially enlarged view showing a linear motor of a furtherembodiment of the present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Preferred embodiments of the present invention will now be describedwith reference to the accompanying drawings. FIG. 1 is a cross-sectionalview of a cylindrical linear motor according to one embodiment of thepresent invention. The linear motor of this embodiment includes a stator1 and a movable element 3 as shown in FIG. 1. The stator 1 comprises anarmature 9 that includes an iron core 5 and a plurality of armaturewindings 7. The iron core 5 has a cylindrical yoke 5 a and a pluralityof magnetic pole portions 5 b extended from the yoke 5 a. An annularslot is formed between two adjacent magnetic pole portions 5 b. Theplurality of armature windings 7 are respectively wound around theplurality of annular slots via insulators in a circumferential directionof the cylindrical yoke. In this embodiment, the plurality of armaturewindings 7 are disposed so that three phases are arranged in the orderof U-phase, −U-phase, U-phase, V-phase, −V-phase, V-phase, W-phase,−W-phase, and W-phase. The flow of magnetic flux from the armature isexplained in detail in Japanese Patent Application Publication No.11-332211 (JP11-332211A) and other documents.

A copper tank 6 a is provided inside the cylindrical yoke 5 a. Coolingwater is supplied to the tank 6 a via a supply pipe 6 b and isdischarged therefrom via a discharge pipe 6 c. Thus the cooling watercirculates in the copper tank 6 a to cool the armature 9.

The movable element 3 includes a cylindrical base 11 made of a softmagnetic material such as silicon steel sheets, and a permanent magnetmagnetic pole row 15 attached onto an inner peripheral surface of thebase 11. The permanent magnet magnetic pole row 15 has a plurality of(nine in this embodiment) permanent magnets P1 to P9. The armature 9 ispartially arranged inside the permanent magnet magnetic pole row 15,which is cylindrical in shape, so that the permanent magnets P1 to P9and the plurality of magnetic pole portions 5 b may face each other. Themovable element 3 reciprocates in an extending direction of the yoke 5 aof the iron core 5 (that is, the moving direction indicated by an arrowD1). The permanent magnets P1 to P9 of the permanent magnet magneticpole row 15, namely, the first to Mth permanent magnets (where M is anatural number of four or more; M is nine in this embodiment) arearranged at a given interval in a row along the moving direction so thatnorth poles and south poles may be alternately arranged. Each of thefirst to the ninth (Mth) permanent magnets P1 to P9 is constituted froma cylindrical permanent magnet. Therefore, the outer outline of thepermanent magnet P1 to P9 (Mth) is a ring in shape. In this embodiment,nine permanent magnets are used in the linear motor. The length of thefirst and the ninth permanent magnets P1 and P9 is half or even lessthan the length of the remaining permanent magnets. For this reason, thefirst and ninth permanent magnets P1 and P9 are counted together as onepole. As a result, the number of poles of the permanent magnet magneticpole row 15 is regarded as eight in total. In a portion of the armature9 that faces the permanent magnet magnetic row 15, the number of slotsis nine. As mentioned above, the current of three phases flows throughthe plurality of armature windings 7. Accordingly, the number of slotsper pole per phase for the linear motor of this embodiment is calculatedas ⅜, three-eighths.

The iron core 5 of the linear motor according to this embodiment isfixed to stator fixtures 17A and 17B at both ends thereof in the movingdirection D1. The stator fixtures 17A and 17B are fixed onto a basestation 19. A guide block 21 is fixed to a lower portion of the movableelement 3, and is slidably supported by a guide rail 23 disposed on thebase station 19. With this configuration, the movable element 3 iscapable of moving in the moving direction D1 with respect to the stator1 as the guide block 21 slides on the guide rail 23.

As shown in FIG. 2, which is a partially enlarged view of FIG. 1, thepermanent magnets other than two permanent magnets disposed at the endsof the permanent magnet magnetic row, namely, the permanent magnets P2to P8 (the permanent magnets disposed in the middle of the permanentmagnet magnetic row as viewed in the moving direction) other than thefirst and ninth permanent magnets P1 and P9 (Mth) in the permanentmagnet magnetic row 15 have the same length W as measured in the movingdirection D1 of the movable element. Hereinafter, the permanent magnetsP2 to P8 are called as middle permanent magnets P2 to P8, and the firstand ninth permanent magnets P1 and P9 are called as end permanentmagnets P1 and P9. The end permanent magnets P1 and P9 have the samelength W1 as measured in the moving direction D1, which is shorter thanthe length W of the middle permanent magnets P2 to P8. The fifth to theninth permanent magnets P5 to P9 are not illustrated in FIG. 2. When apitch between centers of two adjacent magnetic pole portions 5 b of thearmature 9 is defined as τs, the pitch τs is equal to the length W ofthe middle permanent magnets P2 to P8, namely, W=τs. A ratio W1/W of thelength W1 of the end permanent magnets P1 and P9 to the length W of themiddle permanent magnets P2 to P8 is 0.43<W1/W<0.49. When a distancebetween the end permanent magnet P1 and its adjacent middle permanentmagnet P2, and a distance between the end permanent magnet P9 and itsadjacent middle permanent magnet P8 are respectively defined as d_(o)and a pitch between centers of two adjacent permanent magnets among themiddle permanent magnets P2 to P8 is defined as τp, a ratio(d_(o)+W1)/τp of the sum of d_(o) and the length W1 to the pitch τp is0.6<(d_(o)+W1)/τp<0.66.

Next, the relationship between the cogging torque and the ratio W1/W wasstudied by varying the value of W1/W for the linear motor of thisembodiment. For the linear motor used in the test, the length W is 12mm, the pitch τs is 12 mm, the distance d_(o) is 3 mm, the pitch τp is13.5 mm, and the thickness of the permanent magnets P1-P9 is 4 mm each.The ratio W1/W was varied by changing the value of the length W1. FIG. 3shows measurement results. It is obvious from FIG. 3 that the coggingtorque was suppressed in the range of 0.43<W1/W<0.49.

Next, the relationship between the cogging torque and displacement ofthe movable element was studied for the linear motor of this embodimentwherein the ratio W1/W is 0.45, and for a linear motor of a comparativeexample wherein the ratio W1/W is 0.54. Here, both of the linear motorsof this embodiment and the comparative example have the same length W of12 mm, while they differ in length W1. FIG. 4 shows measurement results.It is obvious from FIG. 4 that changes in cogging torque with respect todisplacement of the movable element are smaller in the linear motor ofthis embodiment than in the linear motor of the comparative example.

FIG. 5 shows permanent magnet magnetic pole rows 115 of a linear motoraccording to another embodiment of the present invention. In the linearmotor of this embodiment, each permanent magnet is an arc in shape andthere are eight permanent magnet magnetic pole rows 115. FIG. 5 is anexploded plan view showing half (four) of the eight permanent magnetmagnetic pole rows 115. The linear motor of this embodiment has anotherhalf (four) of the permanent magnet rows 115 which are similar to thoseshown in FIG. 5. As shown in FIG. 5, middle permanent magnets PC otherthan end permanent magnets P11 to P14 and P21 to P24 respectivelydisposed at either ends of the eight permanent magnet magnetic pole rows115, have the same length W as measured in the moving direction D1.

The end permanent magnets P11 to P14 disposed at one-side ends of theeight permanent magnet magnetic pole rows 115 have different lengths W11to W14, as measured in the moving direction D1. The lengths W11 to W14are less than the length W, as measured in the moving direction D1, ofthe middle permanent magnets PC. Further, when an average length of thelengths W11 to W14 of the permanent magnets P11 to P14 is defined asW10, a ratio W10/W of the average length W10 to the length W of themiddle permanent magnets PC is 0.43<W10/W<0.49. Similarly, the ninth(Mth) permanent magnets P21 to P24 disposed at the other-side ends ofthe eight permanent magnet magnetic pole rows 115 have different lengthsW21 to W24, as measured in the moving direction D1. The lengths W21 toW24 are less than the length W of the middle permanent magnets PC asmeasured in the moving direction. Further, when an average length of thelengths W21 to W24 of the permanent magnets P21 to P24 is defined asW20, a ratio W20/W of the average length W20 to the length W of themiddle permanent magnets P2 to P8 is 0.43<W20/W<0.49.

Also, in the embodiment shown in FIG. 5, pitches between centers ofadjacent middle permanent magnets PC are equal to each other. A distancebetween the end permanent magnets P11 to P14 and their adjacent middlepermanent magnets PC, and a distance between the end permanent magnetsP21 to P24 and their adjacent middle permanent magnets PC mayrespectively be defined as do as with the previous embodiment. A pitchbetween centers of two adjacent permanent magnets among the middlepermanent magnets P2 to P8 may be defined as τp. An average length ofthe lengths W11 to W14 of the end permanent magnets P11 to P14 may bedefined as W10 or an average length of the lengths W21 to W24 of the endpermanent magnets P21 to P24 may be defined as W20. Then, a ratio (d_(o)+W10)/τp (a ratio of the sum of the distance d_(o) and the averagelength W10 of four end permanent magnets to the pitch τp) or a ratio(d_(o)+W20)/τp (a ratio of the sum of the distance d_(o) and the averagelength W20 of four end permanent magnets to the pitch τp) may be definedas 0.6<(d_(o)+W10)/τp<0.66 or 0.6<(d_(o)+W20)/τp<0.66.

For the linear motor of this embodiment, a similar test result as shownin FIG. 4 was obtained.

FIG. 6 is a partially enlarged view showing a linear motor of a furtherembodiment of the present invention. In the linear motor of thisembodiment, spacers 201 are respectively disposed between two adjacentpermanent magnets of the plurality of permanent magnets P1 to P9 to formspaces 200 therebetween. Other configuration is similar to that of thelinear motor of FIG. 2. The spacer 201 is formed by cutting an aluminumwire made of a nonmagnetic material having a diameter which is equal tothe width of the space 200.

The present invention is not limited to these embodiments, but variousvariations and modifications may be made without departing from thescope of the present invention.

1. A linear motor which includes a stator and a movable element,comprising: an armature disposed on one of the stator and the movableelement, the armature including: an iron core having three or moremagnetic pole portions arranged along a moving direction of the movableelement; and armature windings of three phases wound around the ironcore to excite the three or more magnetic pole portions; and a pluralityof permanent magnet magnetic pole rows disposed on the other of thestator and the movable element, the permanent magnet magnetic pole rowseach including: a base made of a magnetic material; and a plurality ofpermanent magnets disposed at a given interval in a row along the movingdirection on the base so that north poles and south poles mayalternately be arranged, the plurality of permanent magnets facing thethree or more magnetic pole portions with a given gap therebetween;wherein a plurality of the permanent magnets disposed at one side endsof the plurality of permanent magnet magnetic pole rows are different inlength from one another as measured in the moving direction, and anaverage length of the permanent magnets disposed at the one side ends isdefined as W10; a plurality of the permanent magnets disposed at theother side ends of the plurality of permanent magnet magnetic pole rowsare different in length from one another as measured in the movingdirection, and an average length of the permanent magnets disposed atthe other side ends is defined as W20; the average length W10 and theaverage length W20 are less than a length W of the remaining permanentmagnets as measured in the moving direction; the number of slots perpole per phase is ⅜, three-eighths; and when a pitch between centers oftwo adjacent magnetic pole portions of the plurality of magnetic poleportions constituting the armature is defined as τs, the length W of thepermanent magnet is defined as W=τs; a ratio W10/W of the average lengthW10 to the length W is 0.43<W10/W<0.49; and a ratio W20/W of the averagelength W20 to the length W is 0.43<W20/W<0.49.
 2. The linear motoraccording to claim 1, wherein a spacer, which is made of a nonmagneticmaterial, is disposed between two adjacent permanent magnets of theplurality of permanent magnets to form a space therebetween.
 3. Thelinear motor according to claim 2, wherein the spacer is formed bycutting a wire material.
 4. The linear motor according to claim 1,wherein the permanent magnet magnetic pole rows are disposed on themovable element; the base is cylindrical or columnar in shape, andcommon to all the permanent magnet magnetic pole rows; the plurality ofpermanent magnets are fixed onto an inner peripheral surface of thebase; the armature is disposed on the stator; the stator is partiallyarranged inside the movable element; the armature is fixed onto a basestation via stator fixtures at both ends thereof in the movingdirection; a guide block is fixed to the movable element, the guideblock is slidably supported by a guide rail disposed on the basestation; and the movable element is capable of moving in the movingdirection with respect to the stator as the guide block slides on theguide rail.